The World Federation One Stop Fiqh

Ruling 203

If the blood of a human being, or of an animal whose blood gushes out [when its vein is cut], is sucked by an animal that is commonly known to have no blood, such that it may be absorbed in that animal’s body – like when a mosquito sucks blood from a human being or  an animal – then that blood is pure. This is called ‘transfer’. As for the blood that a leech sucks from a human being for treatment, as it is not known whether or not that blood becomes part of its body, it is impure.